This invention relates to a primer for cartridge casings for projectile ammunition, such as, but not limited to 20 mm Phalanx cartridges used by the military. Technology has long passed beyond percussion initiation of primers for shell cartridges having high rate of fire for military applications. Instead, primers containing pyrotechnic compositions are now electrically initiated. This initiation is accomplished by a relatively simple process of bringing a current-carrying plunger in the gun breech structure up behind the backside of the cartridge casing, once the cartridge is properly chambered, to electrically contact a button electrode therein for completing an electrical circuit through the primer button for initiating detonation of pyrotechnic composition in the circuit path. Detonation of the primer pyrotechnic composition in turn ignites propellant inside the cartridge casing.
Electrical initiation of cartridge primers is now common practice. Usually an explosive pyrotechnic material is made conductive and sandwiched between a pair of electrical conductors inside the primer. Current is passed through the pyrotechnic material from one conductor to the other to generate a temperature rise in the material sufficient to initiate its detonation. The detonating pyrotechnic material in turn ignites propellant in the cartridge casing to propel a projectile forward.
The electromagnetic environment aboard ships and aircraft, as well as on other military equipment, has increased substantially in recent years along with the increased use of electronic equipment. It is in this dense atmosphere of electromagnetic radiation that cartridges having electrically fired primers are stored, handled and used. High-power radar, for example, and communication equipment emit strong electromagnetic fields to the surrounding environment.
The problem of ammunition firing caused by stray or misdirected electromagnetic radiation has long been recognized. Numerous approaches have been proposed to alleviate or counter this condition. Prior art systems for electric fired primers have proposed inductive and capacitive components that form a balanced bridge to shunt unwanted signals. U.S. Pat. No. 3,181,464 employs special conductors, while U.S. Pat. No. 4,304,184 uses one or more inductors and ferrite beads to absorb unwanted current flow. RF attenuation is proposed in U.S. Pat. No. 4,848,233 by providing a conductive path in a special spiral shape with at least one reversal of direction embedded in ferrite material. Still other recent approaches are indicated in U.S. Pat. Nos. 4,893,563; 4,967,665; and 5,036,768. More recently, in U.S. Pat. No. 5,027,707 it is proposed to provide the conductive pyrotechnic mixture with a carbon layer to provide a parallel resistive current path in addition to the path through the conductive mixture.
Stray electromagnetic radiation can be coupled into an electroexplosive cartridge primer to initiate unwanted detonation Propagation of radiated electromagnetic energy into the primer, requires a coupling mechanism from the external environment. Coupling mechanisms generally fall into one of three categories (1) antenna coupling, (2) capacitive coupling, or (3) aperture coupling.
Antenna coupling is a mechanism by which an electrically conductive object exposed to the external radiated electromagnetic environment transforms radiated energy into conducted energy and, if that object is allowed to contact a sensitive system element, transfers this energy to that sensitive element or component. Any electrically conductive object, e.g., wires, tools, human beings, etc., can act as unwanted antennas.
Capacitive coupling requires a conducting object (antenna) in proximity to a sensitive element (e.g., primer button). At frequencies from the upper end of the communications range through the lower end of the radar range, it is possible to couple RF energy by virtue of the capacitance that exists between external objects and the primer button.
Aperture coupling is a mechanism by which radiated electromagnetic energy couples directly from an external environment into sensitive system elements or components via holes, seams, or other openings. Generally, this entry path is more efficient at frequencies where the wavelength is small compared to the dimensions of the opening.